CHENG Jian-qun，RUAN Shuang-chen，GUO Chun-yu，HU Xue-juan，and ZHENG Wan-jun

College of Electronic Science and Technology Shenzhen Key Laboratory of Laser Engineering Shenzhen University Shenzhen 518060 P.R.China

Recently，narrow linewidth lasers have attracted much interest for the potential applications，such as optical fiber sensors，optical measurement，spectroscopy etc.Different materials and approaches have been proposed to operate narrow linewidth lasers.A single-longitudinal-mode(SLM)single-wavelength fiber laser with a fiber Bragg grating(FBG)was realized to measure localized temperature variation［1］.A novel narrow linewidth Bismuth-doped all-fiber ring laser operating at 1 177 nm was reported［2］.Employing a fiber taper，a tunable narrow linewidth Tm-doped silica fiber laser with a narrow linewidth of 148 pm was realized［3］.Utilizing in-line cascade biconical tapers filter，a narrow line-width Tm doped double-clad silica fiber with narrow line-width of ～ 60 pm was achieved［4］.An all-fiber，narrow-linewidth，high power Yb-doped silica fiber laser at 1 179 nm was also demonstrated［5］.

Photonic crystal fibers(PCFs)have obtained wide application in recent years due to their novel optical characteristics with respect to conventional fiber，such as flexible optical waveguide design，low loss，ultrahigh nonlinearity［6］，and controllable dispersion［7］.McNeillie F C et al［8］designed a polarization maintaining， double-clad，Yb-doped photonic crystal fiber and demonstrated its lasing properties.Using four-wave mixing effect in photonic crys-tal fiber，a multi-wavelength bismuth-based Er-doped fiber laser was obtained［9］.With a low-cost microchip laser and a long photonic crystal fiber taper，a supercontinuum source with a very efficient visible conversion was reported［10］.On the basis of two types of hydrogen filled hollow-core photonic crystal fiber，compact UV visible multiline Raman lasers were realized［11］. A passively Q-switched PCF laser with Cr4+∶YAG as a saturable absorber was demonstrated［12］.

To date well-established technique for achieving narrow-line or SLM operation in rare-earth-based fiber lasers has not，to the best of our knowledge，been applied to photonic crystal fiber sources.

In this paper，we report for the first time an all-fiber narrow-linewidth Er-doped silica fiber laser operating at 1 550 nm with only 55 pm linewidth output.To achieve narrow band operation，a couple of matched uniform FBGs are used as wavelength filter in the resonant cavity.The results described in this paper are particularly important for extending the application of the novel narrow-linewidth laser.

1 Experiment

The configuration of the proposed all-fiber narrow linewidth Er-doped photonic crystal fiber laser is illustrated in Fig 1.A 980 nm laser diode(LD)with a maximum output power of 270 mW and a single mode(SM)output fiber is used as pumping source and the emitted pump light is coupled into the fiber laser cavity through a 980/1 550 nm wavelength division multiplex(WDM)coupler.The linear laser cavity is composed of a segment of 9 m long air-clad Er-doped SM photonic crystal fiber used as intracavity gain medium and two matched uniform FBGs(FBG 1 and FBG 2)used as cavity mirrors，which are directly written into a segment of SM fiber core by using the phase mask and UV exposure method，respectively.

Fig.1 Schematic of all-fiber narrow linewidth Er-doped PCF laser圖1 全光纖窄線寬摻鉺光子晶體光纖激光器示意圖

The cross-section micrograph of Er-doped PCF is shown in Fig 2 and has a similar hexagonal holey clad.The core diameter of used PCF is 3.34 μm with numerical aperture of 0.143 and the mass fraction of Erbium ions up to 0.1%.

Fig.2 The cross-section micrograph of PCF圖2 光子晶體光纖橫截面顯微圖

By a precise operation，nearly identical reflection wavelengths of the FBG 1 and FBG 2 can be achieved.The transmission spectrum of them are obtained and shown in Fig 3 and Fig 4 with Advantest Q8384 optical spectrum analyzer(OSA).The central wavelength of FBG 1 and FBG 2 are 1 550.084 nm and 1 550.059 nm，respectively.In the resonant cavity，F(xiàn)BG 1 with reflectivity(R)of 55%and FBG 2 with reflectivity of 99% at the lasing wavelength of 1 550 nm are used as the low reflection mirror and high reflection mirror，respectively.To obtain narrow linewidth lasing，reflection peaks of FBG 1 and FBG 2 have 3-dB bandwidth of 0.093 nm and 0.193 nm，respectively，which is beneficial to reduce mode competition and enhances wavelength stabilization.

An isolator with operating wavelength from 1 535 to 1 565 nm can suppress 4%Fresnel backreflection coming from the fiber end face of output port 2.

Fig.3 Transmission spectrum of FBG 1圖3 光纖布拉格光柵1的透射譜

Fig.4 Transmission spectrum of FBG 2圖4 光纖布拉格光柵2的透射譜

2 Results and discussions

The laser output power is monitored by a power meter，and the output power characteristics of the laser are shown in Fig 5.Power of output port 2 is too low，so we mainly measure the power and spectral characteristics of output port 1.The maximum laser output power was 8 mW at the maximum LD pump power(247 mW).The slope efficiency with respect to the launched pump power is 3.2%and the pump threshold power of the laser is 3 mW.From Fig 5，it can be seen that the output power increases linearly with launched pump power and no evidence of any power limitation due to nonlinear scattering is found in the laser.Because 3 dB bandwidth of FBG 1 and FBG 2 are too narrow，the number of activating ions is smaller and thus the output power is not high.

Fig.5 Output characteristics of the all-fiber narrow linewidth Er-doped PCF laser圖5 全光纖窄線寬摻鉺光子晶體光纖激光器的輸出特性圖

The spectral characteristic of the laser is measured using an optical spectrum analyzer(Yokogawa Company，AQ6370B)with a resolution of 0.02 nm and shown in Fig 6.Its resonant center wavelength is located at 1 550.22 nm，which well coincides with the transmission peak of FBG 1 and FBG 2 shown in the Fig 3 and Fig 4.In order to reduce measurement error of the OSA，10-times continuous measurements were conducted and their arithmetic mean value was calculated.The result shows that the lasing wavelength of the proposed laser has a side-mode suppression ratio(SMSR)of better than 50 dB and 3-dB bandwidth of 55 pm.

Fig.6 Output spectrum of all-fiber narrow linewidth PCF laser圖6 全光纖窄線寬光子晶體光纖激光器的輸出譜

Ten-time repeated scans of the lasing output with 1-min interval are carried out，which is shown in Fig 7.Repeated results indicate that the lasing output is rather stable at room temperature.

Fig.7 Repeated scanning of output wavelength of the PCF laser per minute within 10 min圖7 光子晶體光纖激光器輸出波長在10 min內每隔1 min的重復掃描圖

The output power fluctuation of the laser with time is shown in Fig 8.The average power of the laser keeps very stable with a maximal fluctuation less than 0.07 dB within 12 min.

Fig.8 Power fluctuation of the lasing wavelength of the proposed PCF laser within 12 min圖8 光子晶體光纖激光器的激發(fā)波長在12 min內的功率波動圖

Conclusions

By directly writing two matched FBGs in single mode fiber，a stable and 980 nm pumped continuous-wave all-fiber narrow linewidth Er-doped photonic crystal fiber laser operating at 1 550 nm is proposed and experimentally demonstrated.The slope efficiency of 3.2% and maximum output power of 8 mW with respect to pump power were obtained，respectively.Experimental results show that the maximal output power fluctuation of the laser is less than 0.07 dB within 12 min.Output power is possibly further enhanced by using higher power pump sources at 980 nm.

? This work was supported by Ph D Programs Foundation of Ministry of Education of China(20104408110002).

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